Many current pharmacological approaches to combating Alzheimer's disease (AD) seek to block A? production through inhibition of the amyloidogenic enzymes known as ?- and ?-secretases. An alternative approach is to activate the ?-secretase processing of amyloid precursor protein (APP), which is mediated by several members of the disintegrin family of metalloproteases, ADAM9, ADAM10 and ADAM17. Processing of APP by these ?-secretases is thought to be beneficial with respect to AD since it limits production of A? and generates the neuroprotective soluble APP? (sAPP?) product. Fibulin-1 (Fbln1) is an extracellular matrix protein, expressed in the brain by neurons, that binds the amino terminus of APP and sAPP?. The significance of this interaction is not yet established however, we have found that Fbln1 also binds to other membrane anchored ?-secretase substrates, heparin binding-epidermal growth factor (HB-EGF) and neuregulin-1 (NRG1). We also show that Fbln1 acts to inhibit the proteolytic release of soluble forms of HB-EGF and NRG1. Furthermore, we have found increased levels of sAPP? in the conditioned culture medium of Fbln1 null mouse embryo fibroblasts (MEFs) as compared to wildtype MEFs. Based on these findings it is hypothesized that Fbln1 serves as an inhibitor of ?-secretase processing of APP and therefore may represent a therapeutic target that if inhibited might lead to augmented ?-secretase processing of APP and reduced pathological APP cleavage. To address this hypothesis there are three specific aims: 1) Determine whether brain APP proteolytic cleavage is altered in Fbln1-deficient mice, 2) determine whether transgenic overexpression of Fbln1 accelerates A? production and exacerbates AD pathogenesis, and 3) determine whether Fbln1 inhibits ?-secretase processing of APP in cultured cells.